1. Field of the Invention
Embodiments of the invention pertain generally to the field of ion and/or proton conducting membranes. More particularly, embodiments of the invention are directed to ion and/or proton conducting membranes, devices incorporating ion and/or proton conducting membranes, methods of fabrication of ion and/or proton conducting membranes and devices incorporating same, and applications for said membranes, particularly, but not limited to, fuel cells, gas sensors, and electrocatalytic devices.
2. Description of Related Art
Ion conducting membranes are used in fuel cells, electrochemical membrane reactors, and in chemical sensors. In these exemplary applications, the membrane is electrically insulating but is conductive to protons or oxygen ions. In fuel cells, the membrane performance largely determines the fuel cell operating conditions and, as a result, the design of the entire fuel cell device. The two most common classes of fuel cells are polymeric electrolyte membrane fuel cells (PEMFCs) and solid oxide fuel cells (SOFCs). The ion exchange membranes in PEMFCs are polymers that function most effectively below 100° C. There are no polymeric ion conducting membranes reported that operate effectively above 200° C. The upper temperature limit on polymeric ion conducting membranes means that expensive platinum catalysts should be used for the oxidation and reduction reactions in fuel cells.
The ion exchange membranes in SOFCs are ceramics that operate most effectively at temperatures above 700° C. The temperature is high enough to allow non-precious metal catalysts to be effective for the oxidation and reduction reactions in fuel cells. However, the SOFC operating temperature is sufficiently high that stress from thermal cycles can, and often does, lead to device failure.
There is currently significant interest in developing effective and commercially viable ion conducting membranes that can be used in an intermediate temperature range between about 200-600° C. The discovery of an effective intermediate temperature ionic conducting membrane could truly revolutionize the fuel cell industry. The intermediate temperature range of 200-600° C. would be low enough to allow fuel cell construction using low cost materials, but high enough to use non-precious metal catalysts and allow internal fuel reforming of hydrocarbon fuels.
Previous approaches to creating membranes suitable for the intermediate temperature range have focused on either finding new ion conductors with higher conductivity, or making existing membranes thinner to reduce overall resistance.
Very thin ceramic membranes are too fragile to be self-supporting, and are typically supported in fuel cell devices by either the anode or cathode material. Ito et al., (“New Intermediate Temperature Fuel Cell with Ultra-Thin Proton Conductor Electrolyte” J. Power Sources 2005, vol. 152, pp. 200-203) report a fuel cell that uses a palladium foil hydrogen membrane to support an ultrathin (˜700 nm thickness) BaCe0.8Y0.2O3 ceramic known to exhibit purely protonic conductivity below 600° C. The palladium foil not only supports the thin proton conducting layer, but simultaneously serves as a fuel cell anode and as a hydrogen membrane. After coating the proton conducting layer with a perovskite ceramic cathode, they referred to the resulting three layer structure as a “hydrogen membrane fuel cell” or HMFC, as illustrated generically in FIG. 1. At temperatures above 300° C., hydrogen dissolves into the palladium in the form of protons and electrons. The protons travel through the palladium foil and then through the proton conducting ceramic. Since the proton conducting ceramic is electrically insulating, the electrons are forced to travel from the palladium through an external circuit to the cathode, thereby generating electricity. The reported performance of the HMFC demonstrated a maximum power density of 0.9 W/cm2 at 400° C. and 1.4 W/cm2 at 600° C.
One significant limitation in the HMFC described above is the use of pulsed laser deposition to create the thin film of ceramic on palladium. Pulsed laser deposition is a high vacuum technique that is known to be unsuitable for economically coating large surface areas (the reported HMFC was in the shape of a circle only six millimeters in diameter). The pulsed laser deposition technique is also difficult if not impossible to employ to coat non-planar substrates, such as the interior of a tube, for example.
Zeolite and molecular sieves have been reported in which mass transport occurs through pores in the crystalline framework of the material. Thin zeolite and molecular sieve membranes can be deposited through chemical methods on both planar and tubular supports. Mass transport inside zeolite and molecular sieve crystals is typically anisotropic, with the most favorable mass transport occurring along one crystal axis. Multicrystalline zeolite and molecular sieve membranes with randomly oriented crystal domains are not optimal for mass transport due to the random mass transport path through the membrane and resistance to mass transport at boundaries between crystal domains. Lai et al., (“Microstructural Optimization of a Zeolite Membrane for Organic Vapor Separation” Science 2003, Vol. 300 pp. 456-460 demonstrate enhanced zeolite membrane performance by adjusting chemical synthesis conditions to optimize the membrane microstructure to promote mass transport. The optimized membranes have zeolite crystal domains aligned with the crystal axis giving preferred mass transport oriented normal to the membrane surface. In addition, the crystal domains largely span the membrane thickness to reduce or eliminate resistive boundaries between crystal domains in the direction through the membrane thickness. An analogous microstructural optimization approach has not been extended to ion or proton conducting membranes.
Hydroxyapatite (Ca10(PO4)6(OH)2, or “HAP”) is a type of calcium phosphate that has a hexagonal crystallographic structure, which is thermally stable up to 1400° C. The stoichiometric Ca/P molar ratio is 1.67 for stoichiometric HAP, but the apatite crystal structure can be formed with nonstoichiometric Ca/P ratios and with partial substitution of other ions such as chlorine, fluorine, and yttrium into the crystal framework. High temperature electrochemical investigations have indicated that HAP is proton conductive, with the mechanism of conduction hypothesized to be migration of protons along hydroxyl groups lining the c-axis of the crystals. Since proton conduction occurs primarily along one crystal axis (c-axis) in HAP, it is expected that conductivity will be strongly anisotropic in a single crystal. However, there is no reported study to date of high temperature proton conductivity in singe crystals of HAP due to the difficulty of synthesizing large-sized HAP single crystals.
Ban et al., “Hydrothermal-Electrochemical Deposition of Hydroxyapatite”, J. Biomed. Mater. Res., 1998, Vol. 42, pp. 387-395 and Ban et al. “Morphological Regulation and Crystal Growth of Hydrothermal-Electrochemically Deposited Apatite”, Biomaterials, 2002, Vol. 23, pp. 2965-2972 have reported electrochemical/hydrothermal synthesis of thin films of hydroxyapatite on titanium and stainless steel electrodes to make the metal surfaces biocompatible for orthopedic implants. Similar synthesis of hydroxyapatite crystals onto palladium-based hydrogen membranes, useful for fuel cell applications, has not been reported. Electrochemical growth onto palladium membranes is particularly challenging due to hydrogen embrittlement. Embrittlement refers to the membrane warping and damage that occurs when pure palladium is exposed to hydrogen at temperatures below 293° C. The use of palladium alloys rather than pure palladium mitigates warping to some extent, but does not eliminate issues of hydrogen embrittlement. During hydrothermal-electrochemical synthesis, hydroxyapatite nucleation and growth is driven by a local increase in pH near the cathode that accompanies electrolysis of water. As a result, hydroxyapatite grows only on the cathode, not the anode. Since hydrogen gas is evolved at the cathode during electrolysis, the hydroxyapatite cannot be electrochemically deposited without exposing the palladium membrane directly to hydrogen gas.
In view of the foregoing discussion and the known shortcomings of current technology, the inventors have recognized that improvements to the current state of the art and solutions to the known problems in the art will be beneficial and advantageous. These improvements and solutions will be set forth in the following description of embodiments of the invention, the figures, and as recited in the appended claims.